particular form factor which have better accuracy specifications, but they are
more expensive.

The source code for the program is available at the article link. If you do
not have a PIC programmer or access to one, I will be happy to program one
for you if you send it to me and include an SASE. Email ( k3pto@arrl.net) me
first to get contact information.

Signal Interface (Schematic 2)

This circuit allows the PIC to select between two signal sources: the
output of the DIP meter oscillator and an external input, as well as whether or
not to use the divide-by- 10 circuit. Both signal sources are buffered by
74AHC1G14 Schmitt triggers. These devices have a limited amount of
hysteresis which makes them relatively noise immune. The amplitude of the
hysteresis band can be between 0.5V and 1.5V with a 5V supply. The one
disadvantage to using this device is that the input signal must be large enough
to overcome the hysteresis band. (The oscillator I use has at least 2V peak-to-peak output on all bands.) The resistors on the inputs of these two devices are
an attempt to bias the signals to about the mid-point of the window.

As stated previously, the divide-by- 10 circuit is needed to ensure that the
PIC does not get any frequencies that are too high for its internal counter. The
software always takes a preliminary measurement of the frequency using this
divider with a 10 ms gate time in order to determine whether or not the
divider is necessary. The operation of the 74AC161 is such that when the TC
output goes true (count = 15), the inverted TC signal is fed back to the Parallel
Enable input, setting the counter to six which yields 10 states — or dividing the
input signal by 10. There are three test points available for debugging and/or
monitoring signals.

If you intend to use the
frequency counter function at
frequencies lower than 100 Hz,
you might want to increase the
value of C206. The reactance of a
0.1 µF capacitor at 100 Hz is
close to 16K. This is on the same
order of magnitude as the input
resistance of U205.1 due to the
parallel combination of R204 and
R205 (50K).

Display
Circuit
(Schematic 3)

There is nothing special about
the display circuit except perhaps
that I am using FETs to drive the
common cathodes. The main
reason is that I happen to have
quite a few 2N7002s in my parts
bins. The gate resistors are
probably not needed, but I did
not want to have the possibility of
floating gates before the program